Field of the Invention
The present invention relates to a mass spectrometer.
Description of the Related Art
A mass spectrometer having both a Faraday electrode (Faraday collector) and a secondary electron multiplier as its detectors is known. A mass spectrometer of this type can use the detectors selectively, as appropriate, according to the pressure of the measurement atmosphere, required sensitivity and stability, and the like. Namely, the mass spectrometer can use selectively, as appropriate, a mode (Faraday mode) in which the measurement is performed with the Faraday electrode and a mode (secondary electron multiplication mode) in which the measurement is performed with the secondary electron multiplier.
It is known that when a mass spectrometer of this type is used to perform measurement in a space with a pressure of 1×10−2 Pa or higher, a large amount of vacuum ultraviolet light is generated upon ionization of an analyte gas in an ionization chamber. When the vacuum ultraviolet light reaches the ion detector and generates photoelectrons, the background increases in a mass spectrum obtained as a result of the mass spectrometry in either the Faraday mode or the secondary electron multiplication mode. The higher the pressure is, the more the vacuum ultraviolet light is generated, and the more likely the background is to increase.
In this respect, a configuration is known in which the Faraday electrode is not disposed on an axis of a mass spectrometry unit in addition to the secondary electron multiplier, which is not disposed on the axis. For example, a technology disclosed in U.S. Pat. No. 6,091,068 employs a structure in which an additional electrode is provided on an axis of a mass spectrometry unit to avoid the direct irradiation of a Faraday electrode with the vacuum ultraviolet light.